Generalized flow process considered in Example

Note that not all of these equations are independent. For example, if we sum up the mass balances from Envelopes I, II and III, the overall material balance (Equation 3.19) is obtained. Similarly, the sum of the 

Shell and tube heat exchangers evolved because of the need to prevent contamination of the hot fluid by the cold fluid in many heat transfer operations. In the case of a water cooling tower, where one of the fluids is a gas and the other a liquid, an impervious surface or separation is not necessary because the gas and liquid are. readily separable after mixing and exchanging heat. 

Cooling towers fall under the general category of direct-contact equipment. In direct-contact equipment, fouling resistances are automatically eliminated because a surface is no longer available on which they can form. This is a unique feature because it allows direct-contact equipment to operate indefinitely without interruption in thermal performance. 

The principles of direct-contact heat transfer, treated in the discussions to follow, are presently almost exclusively applied to water cooling and humidification of air. It should be noted, however, that these very same principles can be applied to the cooling or heating of just about any insoluble gas or liquid. 

Suppose we take a sample of bone-dry air at some temperature, Ti, and directly contact it with water until it becomes saturated at the same temperature. The water vapor that enters into the air contains with it its latent heat of vaporization. The vapor pressure of water out of the liquid will be greater than it is in the saturated air, causing vaporization to occur and subsequently increasing the humidity of the air-water-vapor mixture. The process of vaporization ends when the vapor pressure of the water in the air becomes equal to that of the liquid. At this condition the air is saturated. During the air saturation process, isothermal conditions for the water can be maintained if heat is supplied to replace the heat lost from it to the gas as latent heat of vaporization. Thus, heat transfer during the saturation of a gas with a liquid can be accomplished without a temperature differential (although this is rarely encountered). This type of heat transfer phenomenon, better known as diffusional heat transfer, is different from conduction, convection or radiation. 

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Figure 3.4 Generalized flow process considered in Example 1.

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